Although lead has traditionally been used in numerous industrial applications, current regulations have mandated the elimination and/or phase out of lead in most commercial products. These mandates have stimulated new product development based upon lead-free technologies.
Soldering applications, particularly in electronics and vehicle manufacturing, have been heavily impacted by the ban on lead. For example, in response to the European Union's RoHS initiative, solder manufacturers have already switched over 75% of their products from traditional tin-lead solders to lead-free formulations. As a result, it has become increasingly difficult to purchase lead-based solders and systems, leading to significantly increased costs and long lead times. Accordingly, their use is frequently reserved for mission-critical applications in the defense, medical, automotive, space, and oil/gas industries.
Numerous alternatives to traditional lead-based solders have been developed (>300), the Sn/Ag/Cu (SAC) system being among the most widely used, but many have exhibited drawbacks that can make them unsuitable for use in certain applications. For example, SAC solder can be unsuitable for extreme environments such as those found in automotive, military, and space vehicles, where long life and reliability are of significant importance. SAC solder has a significantly higher eutectic melting point (m.p. of ˜217° C.) than does traditional Sn/Pb solder (m.p. of 183° C. for 63/37 Sn/Pb or 188° C. for 60/40 Sn/Pb), thus limiting its use to substrates that are capable of withstanding its relatively high working temperatures for effective processing (approximately 240° C.-270° C.). The need for high performance, thermally stable substrates for use in conjunction with SAC can significantly impact the cost of consumer products relative to those in which lower quality substrates can be used. In addition, silver is a relatively expensive component of the SAC system, and there is presently insufficient worldwide silver production capacity (22,000 tons/year) to allow total replacement of lead-based solders (90,000 tons/year) to take place with lead-free solder alternatives containing significant quantities of silver. Silver prices have also recently been subject to rapid escalation and volatility, which are undesirable features for a commodity material. Still another limitation of SAC solder is that its high tin content makes it prone to tin whisker formation, which can increase the risk of electrical shorting.
Several compositions containing nanoparticles have also been proposed as replacements for traditional lead-based solders. Nanoparticles can exhibit physical and chemical properties that sometimes differ significantly from those observed in the corresponding bulk material. For example, metal nanoparticles that are about 20 nm or less in size can exhibit a fusion temperature that is significantly below the melting point of the corresponding bulk metal, thereby allowing metal nanoparticles to be at least partially consolidated into bulk objects at temperatures comparable to those of traditional lead-based and lead-free solder materials. Copper nanoparticles, in particular, can have a fusion temperature that is comparable to that of the working temperature of traditional lead-based soldering materials and have been extensively studied as an alternative solder material.
A number of scalable processes for producing bulk quantities of metal nanoparticles in a targeted size range have been developed. Most typically, such processes for producing metal nanoparticles take place by reducing a metal precursor in the presence of a surfactant. Metal nanoparticles can then be isolated and purified from the reaction mixture by common isolation techniques. However, the as-produced metal nanoparticles are often prone to clumping and are difficult to directly use. For precision applications such as screen and ink-jet printing, as-produced metal nanoparticles can sometimes be unsuitable for utilization in these techniques, unless utilized in highly diluted form.
When metal nanoparticles are dispersed in a solvent to improve their workability and dispensation properties, further difficulties can be encountered when consolidating the metal nanoparticles into bulk objects, joints, and coatings. For example, if extreme care is not taken during metal nanoparticle consolidation, cracking and void formation can occur due to volume contraction as the solvent and surfactant are removed from the vicinity of the metal nanoparticles. Such cracking and void formation can detrimentally impact the mechanical strength and electrical conductivity of bulk objects and like materials formed from metal nanoparticles.
Although metal nanoparticles have desirable attributes that can make them amenable for use in many different applications, nanoparticle formulations that adequately promote both dispensation and nanoparticle consolidation have yet to be developed. The present invention satisfies the foregoing need and provides related advantages as well.